This invention relates to an apparatus for reducing rotation of an article and, in particular, but not exclusively, to an apparatus for reducing rotation of a super conducting quantum interference device (SQUID) in the earth""s magnetic field. The invention has particular application to airborne transient electromagnetic mineral prospecting. However, it should be understood that the invention has application to environments other than transient electromagnetic mineral prospecting and to payloads other than SQUID magnetometers and, indeed, could be used in many environments in which it is desired to reduce rotational movement of a payload at a particular frequency.
Transient electromagnetic mineral prospecting involves subjecting the ground to a large pulsating magnetic field which is produced by an excitation coil usually mounted on an aircraft. A current is periodically supplied to the aircraft mounted coil for producing the magnetic field. During intervals in which current flows, the ground is magnetised by the magnetic field and during intervals in which no current flows, the ground maintains a residual magnetism which eventually decays. The decay of the residual magnetism is related to the electrical conductivity of the ground and the decaying magnetic field can thereby provide an indication of minerals below the earth""s surface.
In order to detect the decaying magnetic field, the aircraft tows a bird (which is generally in the form of a hollow tear-shaped container). The bird supports a SQUID magnetometer which measures the decaying magnetic field and thereby provides an indication of the material below the surface of the earth in the area being surveyed.
The decaying magnetic field which is of interest is many magnitudes smaller than the magnetic field created by the excitation coil. The field created by the excitation coil is also several magnitudes smaller than the earth""s magnetic field at the location of the SQUID magnetometer. Since the decaying magnetic field is usually detected during periods when the current in the excitation coil is not flowing, the decaying magnetic field can be easily distinguished from the magnetic field produced by the aircraft mounted coil.
In order to distinguish the decaying field from the earth""s magnetic field, electronic circuitry associated with the SQUID magnetometer compensates for apparent changes in the earth""s magnetic field. Linear movement of the SQUID magnetometer through the earth""s magnetic field will not interfere to the same extent with monitoring of the decaying field which is of interest. However, since the SQUID magnetometer in the bird is towed by the aircraft, the bird and therefore the SQUID magnetometer is subject to considerable buffeting and movement. Any rotation of the SQUID magnetometer in the earth""s magnetic field will result in a signal which can be confused with the signal of interest which relates to the decaying field.
The object of the present invention is to provide an apparatus which reduces rotation of an article, such as a SQUID magnetometer relative to a particular datum such as the earth""s magnetic field.
The invention, in a first aspect, may be said to reside in an apparatus for reducing rotation of an article at a desired frequency including:
an article support member for supporting the article, the article support member having an inner shell and an outer shell defining a cavity for receiving a damping liquid;
baffle means within the cavity for damping movement of the support member with respect to the damping liquid; and
suspension means for suspending the support member.
The particular frequency at which it is desired to reduce rotation will depend on the application and payload involved. For SQUID magnetometers supported in a bird, the desired frequency may be approximately 25 Hz. However, in other environments, different frequencies may be of importance.
Preferably the suspension means comprises a plurality of strings each having a first end and a second end, the first end of the strings being coupled to the support member and the second end of the strings being coupled to a respective spring member.
Preferably the support member is substantially spherical in configuration and includes holes through the inner and outer shells through which the strings pass, and an internal anchoring point within the spherical support member for anchoring the first end of the strings.
Preferably the spring members include damping means for damping movement of the spring members.
In one embodiment of the invention, the damping means includes resilient pads. In another embodiment, the damping means includes air bellows.
Preferably the spherical support member includes an internal frame structure for supporting the article.
Preferably the article is a dewar for containing a SQUID magnetometer.
Preferably the support member includes a counterweight for balancing the dewar when supported in the support member.
The invention, in a second aspect, may be said to reside in an apparatus for reducing rotation of an article, at a desired frequency including:
an article support member for supporting the article;
a plurality of strings coupled to the article support member at one end;
a plurality of spring members respectively coupled to each of the plurality of strings at the other end of the strings; and
a holding means to which the plurality of spring members is coupled.
Preferably the article support member comprises a spherical support member having an inner shell and an outer shell defining a cavity for receiving a damping liquid.
Preferably the holding means comprises a bird for towing by an aircraft.
Preferably each of the spring members includes a damping means and preferably the damping means comprises resilient pads or air bellows.
Preferably the article is a dowar for containing a SQUID magnetometer.
In yet a further aspect of the invention, there is provided a bird for towing behind an aircraft to move a SQUID magnetometer over an area to be surveyed, the bird including:
a casing defining an interior space;
a support member located in the interior space for supporting the SQUID magnetometer; and
anti-rotation means for reducing rotation of the support member and therefore the SQUID magnetometer relative to the magnetic field of the earth.
Preferably the support member comprises an inner shell and an outer shell defining a cavity for receiving a damping liquid.
Preferably the anti-rotation means comprises baffle means in the cavity and a plurality of spring members coupled to the caning and a plurality of strings respectively connected to the spring members and to the support member for suspending the support member within the casing.